This invention relates to a lens drive for a still camera using a stepper motor as a drive source. This invention also relates to a lens shutter unit assembled into a camera.
Hitherto, a stepper motor has been used widely as a drive source of a lens drive in a still camera. Basically, rotation of the stepper motor used as a drive source is transmitted via a power transmission train such as a gear train, thereby driving a lens. The drive amount of the lens is controlled in response to the rotation amount of the stepper motor.
The lenses for recent still cameras are designed so that the number of lens stop stages is increased in association with a tendency of multifocal systems and zoom lens systems. The previously known control system of a lens drive using a stepper motor as a drive source generally adopts a comparatively simple technique of supplying a forward pulse in a frequency band in a self-start area to the stepper motor, incrementing a preset counter having a setup value determined corresponding to the photograph subject distance, for example, based on the forward pulse, and stopping the motor when the preset counter counts up. To drive a recent camera lens with the number of lens stop stages increased in such a control system, the time required for focusing is extremely prolonged; this is a problem.
Japanese Patent Kokai Publication No. Hei. 6-258564 discloses a lens drive unit or device in which a lens is moved forwardly and backwardly in an optical axis direction while helically rotating a lens block using a mesh engagement between a female helicoid in an inner wall of a support member and a male helicoid in an outer wall of the lens block. The lens drive unit disclosed in that publication however suffers from the following problem. That is, during the helical motion of the movable side lens block relative to the fixed side support member, the lens block is moved forwardly and backwardly in a state in which an end of the lens block contacting a drive member is protruded from the support member. Accordingly, the area of the mesh engagement is varied in association with the forward and backward movement of the lens block, and a notch must be partially formed in the lens support portion (the female helicoid) to avoid the interference with the protruded end of the lens block. This makes it difficult to stably hold the lens moving backwardly.
It is therefore an object of the invention to provide a lens drive capable of shortening the time required for focusing in a recent still camera with the increased number of lens stop stages. In short, according to the invention, there is provided a lens drive comprising a stepper motor that can be rotated forwardly and reversely; motor control means for controlling driving of the stepper motor; a photographing lens for forming a subject image on a predetermined image surface; lens drive means for driving the photographing lens; and coupling means for coupling the stepper motor and the lens drive means and transmitting the forward operation of the stepper motor, thereby driving the photographing lens from an initial position to a focus position, and transmitting the reverse operation of the stepper motor, thereby driving the photographing lens from the focus position to the initial position. The motor control means comprises start control means for supplying a forward pulse in a frequency band in a self-start area to the stepper motor stopping at the initial position to start forward rotation of the stepper motor; acceleration control means for increasing a forward pulse frequency added to the stepper motor placed in the forward state by the start control means to a through area; constant-speed drive control means for maintaining the forward pulse frequency added to the stepper motor placed in the forward state in the through area by the acceleration control means to the proximity of any desired focus position; deceleration control means for again decelerating the forward pulse added to the stepper motor driven to the proximity of the focus position by the constant-speed drive control means to the frequency band in the self-start area; and stop control means for stopping the stepper motor driven to the desired focus position by the deceleration control means. Preferably, the lens drive of the invention may further include memory means storing control data for operating the start control means, the acceleration control means, the constant-speed drive control means, the deceleration control means, and the stop control means in readable form in response to the target focus position, wherein the start control means, the acceleration control means, the constant-speed drive control means, the deceleration control means, and the stop control means are operated based on the control data read from the memory means. Preferably, the lens drive of the invention may further include urging means for urging the lens drive means from the initial position to the focus position. Preferably, in the lens drive of the invention, the initial position of the photographing lens may be placed at a long-range photograph position. Further, in the lens drive of the invention, preferably the motor control means may further include reverse control means for supplying a reverse pulse in the frequency band in the self-start area to the stepper motor to move from the focus position to the initial position for returning the stepper motor to the initial position.
That is, according to the lens drive of the invention, the lens is driven in the self-start area only at the start time and stop time, but the through area occupies most of lens driving. Thus, if the number of lens stop stages is increased, the time required for focusing can be shortened sufficiently. Particularly, as claimed in claim 2, if the lens drive further includes memory means storing control data for operating the start control means, the acceleration control means, the constant-speed drive control means, the deceleration control means, and the stop control means in readable form in response to the target focus position, and the start control means, the acceleration control means, the constant-speed drive control means, the deceleration control means, and the stop control means are operated based on the control data read from the memory means, optimum control considering torque variation, etc., caused by the lens drive position can be carried out easily.
As claimed in claim 3, if the lens drive further includes urging means for urging the lens drive means from the initial position to the focus position, it is made possible to decrease the torque required for the stepper motor, it is made possible to more speed up the frequencies in the self-start area and the through area, and it is made possible to furthermore shorten the time required for focusing. For the autofocus adjustment mechanism adopting the active distance measuring system, it is known that the distance measuring accuracy is lowered as the longer range side. Generally, in the long range the distance measuring operation is executed more than once, thereby compensating for lowering of the distance measuring accuracy. Thus, there is a trend to increase the total time from distance measuring to focusing particularly on the long range side. However, as claimed in claim 4, if the initial position of the photographing lens is placed at a long-range photograph position, it is made possible to shorten the focusing time in long-range mode and it is made possible to average the total time from distance measuring to focusing regardless of the photographing distance or range.
Another object of the present invention is to provide a lens shutter unit which can move a lens forwardly and backwardly in a stable manner when a lens frame is helically moved.
A lens shutter unit according to the present invention comprises: a shutter blade which is attached to a base plate and can be opened and closed; a support frame being fixed to the base plate and extended in an optical axis direction; a lens frame for holding a lens, the lens frame being helically moved in the optical axis direction in a state in which it is housed in the support frame; a spring being placed between the lens frame and the base plate for urging the lens frame in the optical axis direction; and a lens drive ring being supported on the base plate for rotation in the surrounding of an opening made in the base plate and comprising a protruded piece for engaging the lens frame.
With this arrangement, the lens support portion (the female helicoid) can be dispensed with a notch described in the xe2x80x9cBackground of the Inventionxe2x80x9d section, and the lens frame holding the lens can be accommodated within the support frame. Consequently, the offset of the optical axis in the radial direction can be reduced remarkably, and the quality of the camera can be improved.
As claimed in claim 7, the lens shutter unit is preferably constructed such that the base plate is formed with a gear train for transmitting rotation of a rotor positioned on a side of the lens frame to a rack of the lens drive ring. In this case, the rotor for the motor is arranged on the side of the lens frame, and therefore, the unit per se can be made thin in the optical axis direction. The assembly of the unit into the camera can also be facilitated.
As claimed in claim 8, the lens shutter unit is preferably constructed such that a fix lens concentric with the optical axis of the lens is fitted in the support frame. If this construction is adopted, the lens frame is disposed so as to be accommodated within the support frame, and therefore the forward end side of the support frame can be elongated in the optical axis direction. Consequently, the fix lens can be fitted to the elongated portion. That is, a lens group made up of the movable side lens and the fixed side lens can be assembled into the unit per se.
The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 11-17465 (filed on Jan. 26, 1999), which is expressly incorporated herein by reference in its entirety.